This invention relates to a large antenna system for transmitting and receiving radio waves in a plurality of frequency bands, in which the primary radiators are switched to transmit and receive such radio waves.
Conventional antenna systems employed as satellite communication antennas or large radio telescopes are as shown in FIGS. 1 and 2.
FIG. 1 shows an antenna system in which a beam waveguide system is employed as a primary radiation system and a plurality of horns for many frequency bands are provided. In FIG. 1, reference characters 1a, 1b, 1c and 1d designate horns for radiating radio waves having frequency bands fa, fb, fc and fd, respectively; 2, a sub-reflector; 3, a main reflector; 4a, 4b, 4c and 4d, feeding units provided for the frequency bands, respectively; 6 and 7, radiated beams provided by reflecting the radio wave from sub-reflector 2 and main reflector 3; 8 (indicated as 8a or 8b), 9, 10, 11, 12, 13, 14 and 15, focusing reflectors which are curved mirrors or plane mirrors as shown; and 16, the axis of the main reflector 3.
In the case of frequency band fa, the focusing reflector 8 is retracted so that the radio wave from horn 1a is directed to the focusing reflector 12. The radio wave reflected from the focusing reflector 12 is directed to the focusing reflector 13, where it is reflected. The radio wave thus reflected is further reflected by the focusing reflectors 14 and 15, the sub-reflector 2 and the main reflector 3, and is finally radiated in the form of beam 7. A received radio wave is transmitted to the horn 1a, retracing the above-described path.
In the case of frequency band fb, the focusing reflector 8 is set as indicated at 8a, so that the radio wave from the horn 1b is directed to the focusing reflector 12 after being reflected by the focusing reflector 9 and 8a. Then, similarly as in the case of the frequency fa the radio wave is reflected by the sub-reflector 2 and the main reflector 3 and is finally radiated in the form of a beam 7 from the main reflector 3.
In the case of the frequency band fc, the focusing reflector 8 is set as indicated at 8a, and the focusing reflector 9 is retracted, so that the radio wave of the frequency band fc from the horn 1c is directed to the focusing reflector 10, thus reaching the main reflector 3 through the same path as that in the case of the frequency band fb. Finally, the radio wave is radiated in the form of a beam 7 from the main reflector 3.
In the case of the frequency band fd, the focusing reflector 8 is set as indicated at 8b. The radio wave of the frequency band fd from the horn 1d is directed to the focusing reflector 11, where it is reflected towards the focusing reflector 8b. Then, the radio wave reaches the main reflector 3 through the same path as that in the case of the frequency band fb or fc, and is finally radiated in the form of a beam 7 from the main reflector 3.
In the above-described antenna system, while the antenna rotates around an elevation angle axis E.sub.e, the horns 1a through 1d and the feeding units 4a through 4d are stationary. As a result inspection and maintenance are facilitated. However, the antenna system has certain disadvantages. Since a plurality of focusing reflectors are arranged in association with mechanical means for controlling azimuth and elevation angles, the antenna system is intricate and bulky.
In another type of conventional antenna system, as shown in FIG. 2, a beam waveguide system is not used. Instead, different primary radiators (or horns) are selected for different frequency bands.
In FIG. 2, reference characters 1a and 1b designate horns; 2a or 2b, a sub-reflectors; 3, a main reflector; 4a and 4b, feeding units; 5a, 5b, 6a, 6b and 7, the paths of radio waves radiated by the horns 1a and 1b; 16, the axis of the main reflector 3; and 17, the axis of the horn.
In the case of frequency band fa, the sub-reflector is turned towards horn 1a as indicated at 2a. Therefore, the radio wave from horn 1a is reflected by the sub-reflector (2a) and the main reflector 3, i.e., it is radiated through the path 5a, 6a and 7. A received radio wave reaches the horn 1a retracing the above-described path.
In the case of frequency band fb, the sub-reflector is set as indicated at 2b so as to face the horn 1b.
In the above-described antenna system, the horn axis 17 is offset from the axis 16 of the main reflector 3. That is, the antenna system is a so-called offset type antenna system. The sub-reflector is in the form of a non-rotationally-symmetric (not axially symmetric) mirror surface (even if the main reflector is of an axially symmetric mirror surface). Therefore, a cross polarization is produced by the non-rotationally-symmetric mirror surface. Accordingly, in the use of a circularly polarized wave, the beams of the clockwise and counterclockwise polarized waves which are orthogonal with each other are tilted in the opposite directions, as a result of which so-called "beam separation" is caused. This lowers the accuracy in directivity of the antenna and the gain; that is it degrades the characteristics of the antenna. Furthermore, in the use of a linearly polarized wave, the cross polarization characteristic of the antenna is lowered.